1. Field of Invention
This invention relates to an optical module, especially relates to an optical signal source used in a WDM (Wavelength Division Multiplexing) communication.
2. Related Prior Art
In the WDM communication, the wavelength interval to the adjacent channel is defined to be 0.8 nm. This regulation means that the absolute accuracy superior than ±/−0.1 nm is required for the signal wavelength of respective channel. A semiconductor laser, such as DFB laser diode (Distributed Feedback Laser) and DBR (Distributed Bragg Reflector), is utilized for the signal source of the WDM system.
These feedback lasers have a sharp oscillation spectrum with a typical bandwidth less than 50 GHz. However, since the Bragg grating formed within a laser diode chip solely determines the oscillation wavelength, it would be quite difficult to get the desired wavelength due to the uncertainty of the manufacturing process parameter.
It is also known that the oscillation wavelength of individual lasers can be adjusted by the feedback control after the completion of the production. The method is: 1) dividing the output light from the optical module, 2) monitoring the divided light with a spectrum analyzer, and 3) adjusting the temperature of the laser diode and the injection current to the laser, thus controlling the oscillation wavelength. However, this technique uses the optical spectrum analyzer and is quite impossible to apply to the WDM system, which requires a plurality set of such large-scale equipment for respective optical signal source.
Another example is disclosed in U.S. Pat. No. 5,825,792, in which a parallel plates etalon device is used for the controlling of the oscillation wavelength. In the '792 patent, two optical detectors monitor a divergent light emitted from the back facet of the laser diode through the etalon device placed with an angle for the light. By feed backing the differential signal of two detectors to a temperature of the laser, the oscillation wavelength is effectively adjusted. This method realizes the precisely controlled oscillation wavelength, but requires a precisely adjustment of the rotational angel of the etalon device to the divergent light beam of the laser.